1. Field of the Invention
This invention relates to methods and apparatus for reducing the rate at which fatigue cracks grow in structures and more particularly to a method and apparatus for reducing fatigue crack growth in an aluminum alloy aircraft structure.
2. Description of the Related Art
The aluminum alloy sheet materials used in aircraft structural components are subject to repeated loadings which, in some circumstances, cause cracks to form by the process of metal fatigue. Such cracks grow slowly with increasing time and service, finally reaching a critical length of crack that can cause rapid propagation and catastrophic failure of an aircraft. Load surges such as those that can occur because of turbulent air or impact on landing may have some influence on crack growth, but the main cause of continuing crack growth is the stress produced by pressurization of the aircraft at high altitude.
Government regulations call for the airlines to make regular inspections for the formation and growth of cracks by several means, such as by sight or use of electronic devices. As planes become older, for example after twenty years or more, the number of pressurization and depressurization cycles involved will have been sufficient to produce cracks that will continue to grow at ever increasing rates. These cracks can eventually cause sudden catastrophic failure of a critical part of the aircraft, and in some extreme cases can cause complete destruction of an airborne aircraft. Government regulations call for replacement of parts when an inspection shows that a crack or cracks have grown to what has been determined from experience to be a potentially dangerous length. At present, there is no known method for stopping crack growth or for significantly reducing the rate at which cracks grow.
The present invention provides a relatively simple and inexpensive means for greatly retarding crack growth rates, and in some cases, for actually stopping the growth of a crack in an aluminum alloy sheet material.